Damp stain on chimney breast

[HelenaB]

Can anyone help with suggestions please?
We have a large detached Victorian house. Recently a large damp stain has been appearing on the chimney breast wall in one of the upstairs bedrooms. The fireplace in the bedroom is open but not used (although there is a gas fire used occasionally in the room below which uses the same chimney). The stain comes and goes! It appears when the weather is warm and dampish (but not when it rains!) and I turn on the central heating. Is this a condensation problem in the chimney and if so how do we deal with it? Any suggestions would be gratefully received.

Helena

 

[88v8]

Presumably the chimney is unlined, that is to say there is no flexible liner?
If so, what may be happening... the gas fire burns at a relatively low temperature and as the flue gases rise up the chimney they cool until they reach the point where they condense. Now, in the condensation zone, the inside of the chimney is damp, and has little ventilation because the gas fire substantially closes the opening at the bottom. In time, and it could have taken years, the damp leaches through the lime parging and lime pointing into the plaster.
If the chimney was formerly in use with coal, there will be sooty deposits in the flue, and eventually these too will leach through, then there will be a damp sooty stain which is hard to cover up.

Solution... fit a flue liner, or remove the gas fire and go back to coal, or seasoned wood. If you install an iron fireplace with a closure plate, do not close it, the flue must be ventilated.

Or perhaps the chimney is cold and the stain is as you say condensation. What's the humidity reading in the room?

But I suspect the first scenario is the more likely.

Ivor

 

[stuart45]

A common problem is hygroscopic salts getting into the plaster from the flue. These can attract condensation at even a lower R/H. If this is the problem the plaster needs to be hacked off.

 

[HelenaB]

Many thanks for both posts - the chimney isn't lined and the gas fire is a burner set into the cast iron grate with "look alike coal knobs" so doesn't actually fill the hole so there is ventilation. It is in a dining room which is only used occasionally so is not worth the expense of lining the chimney. I'm trying to keep the fabric of the house warm by using the central heating a bit more which seems to be helping but I think that maybe hacking off and replastering the wall might be the most economic option.

Many thanks for your replies.

Helena